The circularly cylindrical birdcage resonator is a popular radiofrequency (RF) transmission coil in magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS). Such birdcage coils are popular because they are able to produce a highly homogenous, circularly polarized RF magnetic field over a substantial fraction of the coil volume. Also, the circular cross section of gradient and RF coil sets provides satisfactory patient accommodation for most operations. Nevertheless, in certain cases the shape of the body poorly approximates a circular section, for example, the lower abdomen, head, and shoulders. For such applications, it may be advantageous to change the cross-sectional shape of the coil.
Another reason to tailor the coil shape is economic. Increasing the field strength and increasing the bore diameter are conflicting requirements in magnet construction. At any given field strength, smaller bore magnets are cheaper to build, site, and operate. However, when measuring certain body regions that do not approximate a circular shape, such as the shoulders, circular coils present a significant amount of unfilled space about the body being measured, that is, the coil is characterized by a low filling factor. Tailoring the geometry of the coil cross-section for certain body regions may reduce the overall size and increase the filling factor of the coil.
An important consideration in the construction of a birdcage coil is the homogeneity of the magnetic field in the coil. The homogeneity of the field in the coil depends in large part on the azimuthal placement of the longitudinal conducting elements about the inner diameter of the coil. In circular coils, the longitudinal elements are evenly spaced and placement of the elements is relatively straightforward. However, this may not be the case for non-circular coils. Thus, an important consideration in the design of non-circular coils is the azimuthal positioning of the longitudinal elements.
According to an embodiment, a nuclear magnetic resonance coil includes an electrically conductive coil configured for use in a nuclear magnetic resonance device. The coil has a cross-sectional area substantially in the shape of a first Cassinian oval.
According to another embodiment, the electrically conductive coil includes a cylindrical body with a cross-sectional area substantially in the shape of a Cassinian oval, and a conducting shield with a cross-sectional area substantially in the shape of another Cassinian oval is formed around the cylindrical body.
According to another embodiment, the electrically conductive coil includes a number of longitudinal conducting elements coupled to a face of the cylindrical body, extending along a length of the cylindrical body. Each of the longitudinal conducting elements has an azimuthal position on the Cassinian oval corresponding to a conformal transformation of an associated position on a corresponding circular coil.
According to another embodiment, a number of charge storage elements are provided, each charge storage element electrically connected to at least a particular one of the plurality of longitudinal conducting elements.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.